Plastics that can conduct electricity are a technological dream harbored by most research scientists and engineers. In fact, conducting polymers have only emerged in the last decade as a new class of materials with electrical and electronic uses. The synthesis, characterization, and applications of some members of this new class of materials are well known in the literature. They have been described, for example, in Polymers in Electronics, edited by T. Davidson, ACS Symposium Series 242, Washington (1984); in H. G. DeYoung, High Technoloqy, 3(1), 65 (1983); and in A Handbook of Conducting Polymers, Vol. I & II, edited by T. Skotheim, Marcel Dekker, N.Y. (1985).
Some anticipated major applications for this class of materials are redox electrodes for high-energy-density rechargeable batteries, light-weight conductors, wire and cable sheathings, electromagnetic shields for computers and electronic equipment, anti-static packaging materials for sensitive electronic components, electrode materials, Schottky diodes, semiconductor junctions, photovoltaic cells, and detectors.
A number of electrically conductive or semiconductive polymeric materials are known. However, only polymers with conjugated backbone are of particular interest. The conductivities of such materials may be made to undergo a transition from an insulator or semiconductor to metal-like, via a process of chemical or electrochemical doping (oxidation and reduction), utilizing various electron acceptors and/or electron donor dopants. Three main classes of such conjugated polymers have been identified, viz. poly(acetylene) and its derivatives, poly(phenylene) and its derivatives and poly(heterocyclic) polymers and cations. Unfortunately, the electrical conductivities for most of the chemically doped conjugated polymers degrade rapidly when those polymers are exposed to ambient air. The detrimental effects of oxygen and moisture on the physical and electrical properties of these materials have so far prevented them from any practical applications.
The only stable conductive polymers that are known to have been produced so far are the heterocyclic polymers and their cations, particularly polypyrrole (PPY) and its charge transfer complexes, obtained via the process of electrochemical oxidation and polymerization in appropriate electrolytes. However, the conductive polymers prepared by such a process have usually been in the form of thin, brittle films. Furthermore, the electrochemical polymerization process is probably too expensive and inefficient for producing these conductive plastics in bulk quantity. It also involves the use of highly toxic electrolytes and solvents. Thus, an alternative simple chemical method for synthesizing and oxidating heterocyclic polymers in bulk quantities is needed.
Polypyrrole and its derivatives have been synthesized chemically in the presence of an acid or peroxide initiator. The work has been reviewed by G. P. Gardini, Advances in Heterocyclic Chemistry, 15, 67 (1973) The products so obtained were mainly insulating films or powder or particulates with room-temperature conductivity typically in the order of 10.sup.-11 ohm.sup.-1 cm.sup.-1. Besides these films are not completely stable in air, not even at room temperature. Moreover, these products are usually contaminated by the polymerization catalyst or initiator. The initially insulating and oxidative films can be doped with bromine (Br.sub.2) or iodine (I.sub.2) to increase their conductivities to the order of 10.sup.-5 ohm.sup.-1 cm.sup.-1.
More recently, somewhat more conductive polypyrrole has been synthesized in the presence of various Fe(III) oxidants, such as Fe(ClO.sub.4).sub.3 and FeCl.sub.3. However, the polymer so obtained is liable to have been contaminated by a residual amount of iron.